The present invention relates to aerators, and, in particular, to an in-line static mixer for mixing gas with a viscous material to form small gas bubbles. This static mixer can be used for making many different products. The description herein includes a complete description of a machine for making soft-serve frozen desserts which uses this aerator to prepare the liquid dessert mix before putting it into the freezer. Three other patent applications are being filed concurrently for various aspects of that machine under the names "DISPENSER FOR SOFT-SERVE FROZEN DESSERT MACHINE" "FREEZER WITH GROOVED HEAT EXCHANGE SURFACE" and "FREEZER WITH INNER CORE".
Many products require the addition of air or other gas to a viscous liquid. Some examples of food products which require the addition of a gas are frozen custard, margarine, cheese spreads, mayonnaise, mousses, and mashed potatoes. Many non-food products also require the addition of air or other gas to a viscous product.
In the prior art, there has generally not been good control over the amount of gas mixed into the product, and the mixing has been done as a batch process rather than as a continuous process.
For frozen desserts, such as soft-serve frozen custards, the dasher in the freezer rotates at high speeds and is used to whip air into the dessert. The product may be held in the freezer for a long period of time, waiting for a customer to come and buy it. If the product in the freezer has a high fat content and is continuously whipped, the dasher functions as a butter churn, and the product can become solid (like butter), which ruins the product. For this reason, soft-serve machines have had difficulty handling products with high fat content that have to be held in the freezer for an extended period of time.
In prior art soft-serve machines, the liquid frozen dessert mix is poured into a hopper at the top of the machine, and there is a tube in the machine which extends above the liquid level to draw ambient air into the machine as the liquid mix flows by gravity into the freezer. The mixing of air into the liquid is done almost entirely in the freezer, by the whipping action of the dasher. Since there is no control over the residence time of the liquid in the freezer, there is no control over the amount of whipping that occurs. The amount of air drawn in depends upon the flow rate of the liquid, which depends upon the liquid level in the hopper (which varies during the day), the amount of product being dispensed out the front of the machine, the cleanliness of the air tube, and so forth.
Thus, prior art soft-serve machines have provided very little control over the amount of overrun (the amount of gas in the product), and the amount of overrun has been much lower in soft-serve machines than in commercial ice cream machines and has varied from day to day and hour to hour due to this lack of control. Since the amount of overrun in the product greatly affects the quality and the profitability of the product, the manufacturer would like to have tight control over the amount of overrun. Also, the smaller the gas bubbles in the product the better the customer likes the product, so good, consistent mixing of gas into the product is important both from a product quality and a profitability standpoint. This is true in other products, such as whipped cream, mayonnaise, and cheese spreads, as well.
As was explained above, in prior art soft-serve machines, the gas to make the product has typically come from the ambient air. Since the ambient air may have pathogens in it, there is very little control over the bacteria and other pathogens in the product. For this reason, the bacteria count rises as the machines operate during the day, and the machines must be taken apart and washed every night (a very slow, labor-intensive process) to ensure that the product does not contain too much bacteria. Since the cleaning process in prior-art machines is very slow and labor-intensive, many facilities do not clean as often as they should, thereby risking product quality.